1. Field of the Invention
The present invention relates to a multi-antenna system. More particularly, the present invention relates to a method and an apparatus for iteration reduction when precoding to transmit downlink data to two or more users in the multi-antenna system.
2. Description of the Related Art
Research is ongoing to provide various multimedia services including a voice service in a wireless communication environment and to support high-quality and high-speed data communication. A part of the research focuses on a Multiple Input Multiple Output (MIMO) system using a spatial channel. The MIMO technology can offer a high data rate by increasing channel capacity within limited frequency resources using multiple antennas at both a transmitter and a receiver.
In the MIMO environment, representative examples of a linear beamforming technique for multiple users include a Channel Inversion (CI) scheme, a Block Diagonalization (BD) scheme, and the like. However, disadvantageously, the BD scheme uses a great number of transmit antennas at a base station. To address this issue, additional approaches, such as antenna selection or eigenmode selection, are needed.
Unlike the BD scheme, a Coordinated BeamForming (CBF) scheme can transmit data streams smaller than the number of receive antennas of each user to each user. Hence, the CBF can transmit data to multiple users with higher performance under various antenna conditions.
Disadvantageously, the schemes of the related art are considerably complex in their iterative optimization, and are scarcely adopted in practice because the number of the iterations varies per channel realization. In addition, additional analysis is required to determine whether the result of the iteration is always converged regardless of the channel state.
To avoid the iteration, the CBF scheme uses the generalized eigenvector of a postcoding vector. However, the CBF scheme degrades the performance when the number of antennas is greater than a certain number. Practically, it is difficult to apply the CBF scheme in a typical communication system including multiple users and multiple antennas.
As discussed above, the continuous iterative optimization of the schemes of the related art involves a great deal of complexity and a different complexity per channel realization, and thus it is difficult to adopt the schemes of the related art in a real world system.
Hence, what is needed is a precoding method that is less complex and uses constant computations at all times.